As a part of the payload of the first European polar platform, the GOMOS instrument has been proposed by a group of 25 scientists from six countries. It consists of a telescope feeding two spectrographs and mounted on a dedicated steerable platform. The transmittance of the atmosphere between 250 and 950 nm is measured by comparing the spectrum of a star outside the atmosphere, and through it. The ozone tangential column is determined from its UV and Chappuis band absorption. This self-calibrated method is particularly well suited for the study of the long-term ozone trend. The altitude of each single measurement is precisely known (+/- 50 m), independently of attitude uncertainties. The instrument, as defined in the course of an industrial phase A study, is presented in some detail. About 25 stellar occultations per orbit, and 350 per day, spread over all latitudes can be performed from 90 km down to 15-20 km of altitude. NO2, NO3, H2O, T(z), and aerosols are also simultaneously determined, important parameters associated to the ozone equilibrium. Over a period of 4 years in orbit, GOMOS should be able to detect a linear decrease trend of 0.05% per year in 10 bands of latitude.